A coastal sandy beach is formed due to sedimentation of sands supplied from a river of land and a coastal sand dune, and the like. In a natural state, the sandy beach is maintained while a phenomenon in which the supplied sands are accumulated on a coast is balanced with a phenomenon in which sands are lost toward the sea by a wave.
Meanwhile, the coastal sandy beach is becoming important as a tourism resource due to environmental conservation as well as the popularity of various marine sports. However, since a riverside is reclaimed or blocked by a concrete embankment due to a river maintenance project, a natural green area is reduced due to an urban facility construction, and a coastal sand dune is reclaimed to be disappeared, an amount of sands introduced into the sea is abruptly reduced, resulting in a reduction in the coastal sandy beach.
Therefore, various methods of preventing the reduction in the coastal sandy beach have been attempted. In examples of such a method, there is a so-called wave breaking method in which an artificial structure is installed to block or attenuate influence of a wave, and there is an artificial nourishment method in which sands of other regions are transported and supplied to a sandy beach.
Here, the wave breaking method is classified into a submerged dike method in which a wave is suppressed by installing an underwater structure on a seafloor away from the coast to the sea and a revetment method in which influence of a wave is prevented from reaching a land side by installing a structure in the vicinity of the coast or inside the coast.
The submerged dike method is classified into, according to a shape of a structure, a method in which only a high wave is blocked using a structure, i.e., a submerged dike installed in an underwater (see FIG. 1) and a method in which all of a low wave and a high wave are blocked using a structure, i.e., an offshore breakwater protruding from a water surface by a certain height.
Meanwhile, in the related art, in the case of the wave breaking method, erosion of a coast is prevented to a certain degree. However, phenomena such as scouring of a beach or an advance and a retreat of a coastline are caused according to the shape and position of the structure, resulting in deformation of the coastline.
In addition, since the submerged dike and the offshore breakwater require enough rigidity not to be swept away by a sea current or a wave, most of the submerged dike and the offshore breakwater are formed as a concrete structure so as to satisfy such a condition. Such underwater structures do a disservice to sailing of a vessel and also have an adverse effect on activation of marine sports.
According to the revetment method, since a wave is pushed to a revetment and generates erosion of an entire surface of the revetment, it is impossible to prevent loss of sands.
In the artificial nourishment method of artificially introducing sands, since many costs are required for collecting and transporting sands and the artificial nourishment method is not a fundamental measure to prevent loss of sands, there is a burden of continuously supplying the sands.